Can I get an ENERGY STAR® certification for a solar thermal collector?

No. ENERGY STAR certification under the Residential Water Heater Program is only available for complete solar water heating systems. The entire system must be certified under the OG-300 program to qualify for ENERGY STAR. And since OG-300 systems must make use of a solar thermal collector certified as a component to OG-100, all ENERGY STAR certified water heaters do include OG-100 certified collectors. But ENERGY STAR certification is not available for the collector alone.